1. Field
The present invention relates to gas turbine engines, and more specifically to a removably attachable snubber assembly for a turbine blade.
2. Description of the Related Art
In an industrial gas turbine engine, hot compressed gas is produced. The hot gas flow is passed through a turbine and expands to produce mechanical work used to drive an electric generator for power production. The turbine generally includes multiple stages of stator vanes and rotor blades to convert the energy from the hot gas flow into mechanical energy that drives the rotor shaft of the engine. Turbine inlet temperature is limited to the material properties and cooling capabilities of the turbine parts. This is especially important for upstream stage turbine vanes and blades since these airfoils are exposed to the hottest gas flow in the system.
A combustion system receives air from a compressor and raises it to a high energy level by mixing in fuel and burning the mixture, after which products of the combustor are expanded through the turbine.
Since the turbine vanes and blades are exposed to the hot gas flow discharged from combustors within the combustion system, cooling methods are sometimes used to obtain a useful design life cycle for the turbine blade or vane. Blade and vane cooling is accomplished by extracting a portion of the compressed air from the compressor and directing it to the turbine section, thereby bypassing the combustors. After introduction into the turbine section, this cooling air flows through passages formed in the airfoil portions of the blades and vanes.
Gas turbines are becoming larger, more efficient, and more robust. Large blades and vanes are being produced, especially in the hot section of the engine system. Of particular challenge is the last stage blade. Traditionally the last stage blade has been solid, tip shrouded and uncooled. This configuration has limitations as the blades require more robustness as the gas path diameters increase and the gas path temperatures increase.
In order to allow for increasing the gas path diameter, turbine blades may be hollow, cooled, curve root attached blades with integral part span snubbers as seen in FIGS. 1 through 3. FIGS. 2 and 3 shows multiple blades and snubbers in a conventional configuration. There is an extensive amount of time and effort involved with making sure the snubbers are properly aligned and within a certain distance in order to make eventual contact once the turbine blades are moving at a certain speed. The turbine snubbers are also used for the purpose of damping blade mechanical vibrations, particularly for blades having high aspect ratio. An edge from each snubber is a contact surface for the next snubber once the turbine blades are moving at a specific rotational speed.
The snubber creates an assembly challenge due to the potential interference between the attachment engagement and snubber contact surface engagement as can be seen in FIGS. 1 and 2. The assembly cannot be completed without specialized tooling and processes. The existing snubber systems are all axisymmetric forming a constant radius, segmented support ring for the large blades. The damping design criteria for bearing angle and contact pressure are often difficult to achieve. The snubber also presents a performance challenge due to the increased aerodynamic blockage as well as its effect on the exhaust diffuser behavior. Assembly issues in regards to the snubbers and the eventual connections are difficult, especially since contact is not made until the turbine blades have started to move at a certain rotational speed.
In current assemblies, the snubbers will connect with each other once the turbine is up and running at a particular rotational speed. The blade untwists as a functional of rotational speed. The airfoil will also have radial growth at increased rotational speeds. Once the blades have sufficiently untwisted, the snubbers come into contact through their contact surfaces once that particular rotational speed is met as shown in FIG. 2. The contact surface, or bearing surface, on each snubber connects once a certain rotational speed is reached. There is typically no initial contact of the snubbers at zero and low speeds as is shown in FIG. 3.
Aeromechanical systems for snubbers and tip shrouds consider the vibratory mode shape to define the bearing angle and contact pressure to achieve the required level of mechanical constraint and damping. Additionally, these systems try to achieve minimum weight to reduce the stress on the blade. There has been no significant reduction in the snubber aerodynamic penalty because of the mechanical requirements for blade support and part life.